Fundamentally, it's because the "marketplace" is owned, operated and funded by the seller. So, they fill it with ads and dark patterns to get you to buy what they want you to buy, which is probably some high-margin junk, and not what you want to buy.

If there were a hypothetical "marketplace" that was operated by, say, a consumer co-operative, funded by subscriptions from consumers, then it would operate with different incentives. It would be something like... Consumer Reports. But, it turns out, people would rather scroll through ads than pay for that kind of service with money, so that's why we have what we have.

Even a traditional marketplace owned by a municipality, or an association of vendors, or a shopping mall, is a bit more consumer friendly because they impose some standards on the tenants/vendors. A true "marketplace", like a stock market, or a fruit and vegetable market, needs to be vendor neutral, and there aren't any "marketplaces" like that on the internet.